1. Field of the Invention
The invention relates to a pickup device for plucked string instrument capable of detecting, as electrical signals, vibrations of strings that occur when an acoustic plucked string instrument such as a guitar, a mandolin, a ukulele is played, and to a plucked string instrument including the same.
2. Description of the Related Art
Generally, a plucked string instrument such as a guitar, a mandolin, or a ukulele is structured such that both ends of a plurality of strings are fixed to an instrument main body constituted of a body and a neck, and a bridge is sandwiched by the strings therebetween and a top of the body in the instrument main body. A plucking operation of plucking the strings between the bridge and a nut on the neck side with fingers or nails produces musical sound. In this plucking operation, the strings are pressed against a fingerboard provided in the neck, and the pitch of the musical sound depends on the pressed position of the strings. The bridge, in a case of a guitar, is formed of a bridge fixedly attached on the top of the body and a saddle supportedly fitted in the bridge.
An electric plucked string instrument is one of such plucked string instruments. The electric plucked string instrument produces musical sound similar to that of an acoustic plucked string instrument in such a manner that a pickup device detects vibrations of strings plucked with fingers or nails to transduce the vibrations into electrical signals, and the electrical signals after being amplified are subjected to filtering or the like to become musical sound signals, which are then electroacoustically transduced by a speaker. According to this electric plucked string instrument, it is possible to play the instrument with a desired volume, to practice playing the instrument while listening to the musical sound through a headphone instead of the speaker, and to record signals of the musical sound produced by the played instrument to a recording medium such as a recording tape or a recording disk.
A conventionally available example of the aforesaid pickup device used for a guitar is a magnetic pickup device that electrically detects vibrations of strings, which are all steel strings, by an electromagnetic induction effect of a coil. However, recent years have seen a demand for faithful detection of raw sound peculiar to the instrument, also in an acoustic guitar, a ukulele, and the like using nonmetallic, nylon strings or the like.
As a pickup device used for this purpose, for example, U.S. Pat. No. 5,123,325B and JP H7-152378A disclose pickup devices using a piezoelectric transducer made of a polymer piezoelectric element or a piezoelectric film.
The pickup devices disclosed in these patent documents are both structured such that a piezoelectric transducer is sandwiched by a bridge and a saddle. The piezoelectric transducer receives vibrations of strings via the saddle to generate an electromotive force by a piezoelectric effect, so that the vibrations of the strings can be detected as electrical signals.
A structure of the conventional pickup device will be simply described with reference to FIG. 11.
FIG. 11 is a cross-sectional view of the bridge of the pickup device taken along a longitudinal direction. This pickup device 35 is structured such that, similarly to a typical guitar, a saddle 16 is supportedly fitted in a bridge 25 fixedly attached on a top 4 of a body to support a plurality of strings 3, and a piezoelectric transducer 36 is inserted in a part where the saddle 16 and the bridge 25 are fitted to each other. The piezoelectric transducer 36 is made of a polymer piezoelectric element or a piezoelectric film having electrodes on upper and lower surfaces thereof and has an elongated shape so as to extend along the whole length of a bottom of the saddle 16. The vibrations of the strings 3 caused by a plucking operation varies a pressing force that is applied to the piezoelectric transducer 36 via the saddle 16, and thus an electromotive force by a piezoelectric effect is generated, so that the piezoelectric transducer 36 outputs electrical signals according to the vibrations of the strings.
However, in the conventional pickup device 35 as structured above, the piezoelectric transducer 36 is constantly given a strong static pressure from the saddle 16 in a state in which the strings are stretched and tuned so as to be playable. The piezoelectric transducer receiving such a preliminary stress needs to be given a considerably strong force in order to generate a sufficient electromotive force. In addition, the piezoelectric transducer 36 generates a strong electromotive force when being displaced in its thickness direction. However, the vibrations of the strings 3 caused by the plucking operation in a guitar or the like is in a direction substantially parallel to the longitudinal direction of the piezoelectric transducer 36 as arranged above. Therefore, a vibration component in the thickness direction is relatively small.
Further, the aforesaid preliminary stress applied to the piezoelectric transducer 36 inhibits free movement of the piezoelectric transducer 36. Therefore, in such a conventional pickup device 35, only small outputs of the electrical signals are obtained from the piezoelectric transducer 36, and thus a preamplifier or the like with a high amplification factor has been indispensable.
Further, in the piezoelectric transducer 36 in the state of receiving the aforesaid preliminary stress, a frequency characteristic of the piezoelectric effect for the vibrations of the strings is inferior, resulting in nonuniform power generation output for frequency components of the vibrations of the strings producing musical sound. Consequently, musical sound produced based on the electrical signals detected by the pickup device 35 becomes far different in tone from the original raw sound of the instrument. Therefore, tone correction by an equalizer has been necessary.